It is known in prior art that an adapter is directly screwed on a wire collecting frame, referring to FIG. 1. Four screws 111, 112, 113, 114 coupled with four metal cylinder screw holes 131, 132, 133, 134 on the wire collecting frame 12 lock the adapter 10 to the wire collecting frame 12, where the wire collecting frame 12 is made of metal and the four metal cylinder screw holes 131, 132, 133, 134 are locked to the wire collecting frame 12 with screws 141, 142, 143, 144.
FIG. 2 shows another fixing device known in prior art where a top cap 21 covers and is fastened with the adapter 20 by screwing the screws 221, 222 to the screw hole seat 211, 212 on the top cap 21 through the screw holes 201, 202 on the adapter 20. Then the adapter 20 and the top cap 21 are secured to the wire collecting frame 23 with the screws 241, 242, 243, 244 screwed to the screw holes 231, 232, 233, 234 on the wire collecting frame 23 and the screw holes 213, 214, 215, 216 on the top cap 21.
As seen from FIGS. 1 and 2, eight and six screws are respectively required and the assembly in FIG. 1 additionally needs four metal cylinder screw holes, which results in not only higher cost but also an inconvenience of the fact that it will not work when losing one or two screws. Besides, screws will be all over the wire collecting frame panel which is not nice to look at.